<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>flex</title>
    <style>
      * {
        margin: 0;
        padding: 0;
      }
      .container {
        display: flex;
        height: 100vh;
      }
      .main {
        flex: 1;
        background: #0b8e61;
      }
      .left{
        flex:0  1 100px;
        order: -1;
        background-color: #fedd00;
      }
      .right{
        flex: 0 1 100px;
        background-color: #fedd00;
      }
    </style>
  </head>
  <body>
    <div class="container">
      <div class="main"></div>
      <div class="left"></div>
      <div class="right"></div>
    </div>
  </body>
</html>
